This invention relates to a magnetic core used in AC electrical equipments such as electric motors, transformers and reactors which are supplied with an output voltage from an inverter as an AC power supply producing a non-sinusoidal wave voltage.
Conventionally, a magnetic core used in AC electrical equipments has been formed by laminating silicon steel sheets into a laminated core or by combining steel sheets having lower ratio of content of carbon with silicon steel sheets into a laminated core. Japanese Laid-open Patent Application (Kokai) No. 51-136108 discloses such a laminated core as described above.
Inverters have recently been employed as high frequency power supplies to run various electrical equipments. Particularly, when a PWM inverter is employed as such a high frequency power supply, an output voltage contains a large amount of harmonics as well as a fundamental wave ranging between 3 and 80 Hz. Consequently, magnetic fluxes passing through magnetic cores of an electrical equipment contains a large amount of harmonic fluxes, which cause magnetostriction in the magnetic cores of the electrical equipment. The magnetostriction causes electromagnetic noise and vibration.
In a variable speed drive of an induction motor, power supply voltage and frequency applied to the motor are controlled by an inverter. However, it is well known in the art that in the case of driving the motor energized from the inverter, an amount of electromagnetic noise or vibration is increased as compared with the case where the motor is energized from a commercial AC power supply. A major cause for this is the harmonics contained in the applied voltage. The voltage and current waveforms produced by the inverter are non-sinusoidal and contain a relatively large amount of harmonics as well as the fundamental wave in the variable frequency range of 3 to 80 Hz while those of the commercial AC power supply are sinusoidal. When the motor is driven by the inverter such as described above, the harmonics cause harmonic fluxes around the motor windings. The harmonic fluxes induce magnetostriction in a stator core and a rotor core. Furthermore, since a transformer and a reactor are necessarily connected to a high frequency circuit in the electrical equipment employing the inverter as the power supply, the electromagnetic noise or vibration emanating from the transformer and reactor is also increased. To reduce an amount of such an electromagnetic noise, an inductor has conventionally been provided between the motor and the inverter so that the ratio of the harmonics to the fundamental wave contained in the current flowing into the motor windings is reduced, thereby reducing the electromagnetic noise.
However, since the inductor is provided between the motor and the inverter, not only the harmonics but the fundamental wave are reduced. Consequently, operating characteristics of the motor such as the power factor or starting torque are varied or lowered. Furthermore, since the inductor is separately necessitated, a space for mounting the motor needs to be increased.